Frequency Modulation (PCOMS 5)

Frequency Modulation (FM)

A modulation process where the carrier frequency varies in proportion to the amplitude of the modulating signal while the carrier amplitude remains constant.

Carrier frequency increases with signal amplitude

As the amplitude of the modulating signal increases, the carrier frequency increases proportionally.

Frequency deviation (fd)

The amount of change in carrier frequency produced by the modulating signal.

Frequency deviation rate

The number of times per second the carrier frequency deviates above or below its center frequency.

Frequency-Shift Keying (FSK)

A type of FM used to transmit binary data in digital cell phones and low-speed computer modems.

Phase Modulation (PM)

A modulation method where the phase of a constant-frequency carrier is varied according to the amplitude of the modulating signal.

Phase shift

A time separation between two sine waves of the same frequency; the greater the modulating signal amplitude, the greater the phase shift.

Maximum deviation in PM

Occurs when the modulating signal is changing at its most rapid rate.

Frequency deviation in PM

Directly proportional to both the amplitude and the frequency of the modulating signal.

Frequency-correcting network (1/f filter)

A low-pass RC filter used to make PM compatible with FM by attenuating high modulating frequencies.

Indirect FM

FM generated using a phase modulator with a frequency-correcting network.

Phase-Shift Keying (PSK)

A modulation method where the phase of a carrier changes according to binary data; also known as Binary Phase-Shift Keying (BPSK).

Side frequencies (sidebands)

Frequencies produced at the sum and difference of the carrier and modulating frequencies.

Modulation index (mf)

The ratio of frequency deviation to modulating frequency, expressed as mf = fd / fm.

Standard FM broadcast limits

Maximum deviation = 75 kHz, maximum modulating frequency = 15 kHz, giving mf = 5.

Bessel functions

Mathematical functions used to determine carrier and sideband amplitudes for various modulation indices in FM.

Narrowband FM (NBFM)

An FM system with a modulation index less than π/2 (≈1.57), conserving bandwidth but reducing signal-to-noise ratio.

FM signal bandwidth

Increases with the modulation index because more significant sidebands are produced.

FM bandwidth formula

BW = 2fmN, where N is the number of significant sidebands (those above 1% amplitude).

Noise

Unwanted interference from lightning, motors, or other sources that produce transient voltage spikes.

Limiter circuit

A circuit in FM receivers that clips amplitude variations without affecting frequency information, improving noise immunity.

Preemphasis

A technique in FM transmitters that amplifies high-frequency components of the signal to improve signal-to-noise ratio.

Deemphasis

A receiver technique that attenuates high-frequency components, restoring a flat frequency response after transmission.

Purpose of preemphasis and deemphasis

To improve the signal-to-noise ratio for high-frequency components during FM transmission.

Capture effect

An FM receiver's ability to lock onto the stronger of two signals on the same frequency, rejecting weaker ones.

Advantages of FM

Better noise immunity, capture effect, and higher transmitter efficiency due to constant amplitude.

Disadvantages of FM

Requires more bandwidth and more complex modulation/demodulation circuitry.

Reason FM is widely used today

Integrated circuits (ICs) have simplified and reduced the cost of FM and PM circuits.